1. Field of the Invention
The present invention relates to an improved method for oil recovery from wells characterized by having high brine concentrations. The invention comprises the injection of microemulsions of specific surfactants which have been found to result in effective and efficient oil recovery.
2. Description of the Prior Art
The present invention relates to the displacement of oil from the pores of subterranean, oil-containing reservoirs, and more particularly to special surface active systems to be used with "waterflooding" techniques to improve the oil displacement efficiency of waterfloods.
Knowledge is wide spread in the oil industry that the so-called "primary recovery" techniques, which include natural flow, gas lifting, gas repressurization and pumping methods, leave substantial quantities of oil in oil-bearing reservoirs. In addition, there are oil-bearing reservoirs, which even though containing large quantities of oil, are incapable of being produced by primary recovery techniques. Recognition of the large amount of residual oil in many oil-producing reservoirs has led to the use of the so-called "secondary recovery" techniques which have as their primary purpose the economical recovery of additional quantities of the residual oil known to be present in the reservoir.
Probably, one of the more common secondary recovery techniques is the so-called "waterflooding" method in which aqueous fluids are injected at one point in the reservoir at pressures sufficient to be forced out into the reservoir and toward a spaced production well or wells, thereby, in effect, displacing it from the pores of the reservoir and driving the oil ahead of the water front.
However, waterflooding is only advantageous when the cost of injecting water and necessary chemical modifiers is less than the value of the oil recovered. Therefore, the displacement efficiency of waterfloods has been the determining factor of whether such a technique will be used.
Generally, the difficulty with waterfloods is that the small pores and capillaries of the reservoir contain hydrocarbons (oil and/or oil and gas) which are generally water immiscible. The existence of high interfacial tensions between the boundary of the water and hydrocarbons seriously impedes the ability of the water to displace oil trapped in the reservoir by capillarity.
Since in many oil reservoirs the oil tends to be trapped within the pores of the rock formations by capillarity, merely forcing water therethrough will not displace much of this trapped oil. However, a reduction in the interfacial tension between the water and the oil will tend to increase the amount of oil that will be displaced by the water. Thus, various aqueous surfactant systems have been proposed for use in waterflooding processes for recovering oil. Surfactants which have been used in the prior art include various nonionic, cationic, and anionic surfactants. Examples of such prior art surfactants include sodium glyceryl monolaurate sulfate, dihexyl sodium succinate, hexadecylnaphthalene sulfonate, diethyleneglycol sulfate, glycerol disulfoacetate monomyristate, p-toluidene sulfate laurate, p-chloroaniline sulfate laurate, sodium sulfato oleylethylanilide, triethanolamine myristate, N-methyltaurine oleamide, pentaerythritol monostearate, polyglycerol mono-laurate, triethanolamine oleate, morpholine stearate, hexadecyl trimethylammonium chloride, ditetradecyl dimethyl ammonium chloride, n-dodecyl-diethyleneglycol sulfate, monobutylphenyl phenol sodium sulfate, and triethanolamine laurate or triethanolamine oleate.
It has been found that the solubilization of oil in aqueous solutions can occur, to an appreciable extent, in an aqueous solution of a surface-active chemical, whenever micelles are formed. But the amount of oil solubilized by a surfactant solution is usually small, because the micelles are very small, comprising about 70 molecules of surfactant, for example. In most instances, an increased concentration of surfactant is ineffective to increase the size of individual micelles. Thus, increased concentrations of surface-active agents are usually ineffective to improve the efficiency with which residual oil is displaced from a porous reservoir.
Under certain conditions, however, with selected surfactants, the micelles can be caused to grow to much larger dimensions, with a consequent increase in the amount of oil solubilized by a given amount of detergent. These systems, which include usually large micelles and large amounts of solubilized oil and aqueous medium, are transparent and are known as microemulsions. It has now been found that specific surface-active agents capable of forming a microemulsion with oil have an unusually high displacement efficiency in the recovery of reservoir oil.
With few exceptions, the formation of a microemulsion requires the presence of an organic polar material such as an alcohol, for example. It has been known in the prior art to displace reservoir oil with a bank of an oil-miscible liquid, for example with alcohol alone, whereby a true molecular solution of the reservoir oil is formed. The microemulsion-forming solutions of the present invention are as effective as alcohol alone in their ability to recover oil, while possessing the distinct advantage of being far less expensive. Frequently, the solutions of the present invention are even more effective than an oil-miscible solvent, for example, an alcohol bank injected by itself, because relatively small volumes of connate water are sufficient to cause a separation of phases when flooding with an alcohol alone (or alcohol solutions of prior art surfactants) whereas the solutions of the present invention retain their oil-miscible character dispite substantial dilution by connate water or brine. However, even should the emulsion break, the resulting mixture will recover the crude.
Applications of the aforedescribed microemulsion compositions of the past have been generally satisfactory when employed in reservoirs of low salinity or, alternatively, reservoirs which may be preflushed with water prior to surfactant injection in order to lower the salinity of the reservoir. Such preflushing to lower salinity levels is difficult and very expensive. Typically such high salinity reservoirs have salinity levels on the order of about 4 to about 5% and sometimes above. At such brine concentrations, the typical prior art microemulsions of surfactants which are employed result in the surfactants precipitating out, rendering the injection of the chemical slug ineffective. There are many reservoirs of such high salinity levels from which tertiary oil cannot be recovered utilizing prior art techniques. It is the purpose of the present invention to provide a method whereby tertiary oil may be recovered efficiently and effectively from such high salinity level reservoir fields, without the necessity of employing a preflush operation.